The present invention relates generally to optical disk drives for reading and recording data on optical disk media, and more particularly to an electromagnetic apparatus for the dynamic positioning of an objective lens element in an optical disk drive.
In an optical disk drive apparatus using laser light, a laser beam is focused on a recordable surface of an optical disk by an objective lens which traverses the span of the disk in a radial direction simultaneously with relatively high speed rotation of the disk. In this manner, a spiral pattern of closely spaced tracks is formed on the disk to represent recorded information. A comprehensive description of optical data recording techniques is found in Marchant, Optical Recording (Addison-Wesley 1990), the complete disclosure of which is incorporated herein by reference.
For maximum storage capacity on the disk, it is desirable to form the information tracks on the disk to have a narrow width and a narrow center-to-center spacing between radially adjacent tracks. Accordingly, for both reading and recording, it is imperative that the laser light beam is accurately and precisely positioned and focused upon a particular track on the disk.
Typically, the objective lens is moved radially from track to track by a coarse positioning actuator. After the lens is coarsely positioned, a fine-positioning actuator moves the objective lens very slightly in the radial (tracking) direction to precisely maintain position over a particular track. The fine-positioning actuator will further position the lens in the vertical (optical or focusing) direction to maintain the proper focus of the light beam on the information-carrying surface of the disk. The fine positioning actuator commonly drives the objective lens in the tracking and focusing directions by the interaction of two electro-magnetic drive coils and a common magnetic field produced by a magnetic circuit, one fixed to the lens holder element and one mounted on the coarse positioning actuator housing, or carriage. The force created by the cross product of the active portions of the coils and magnetic field must be relatively large due to the weight of the moving elements, as well as the precision, and acceleration required in properly positioning the objective lens.
Usually, the objective lens and fine positioning actuator are mounted on a carriage movable in the radial direction by the coarse positioning actuator. An important factor in the performance of such coarse positioning actuators is, therefore, the size and mass of the fine-positioning actuator. Some known fine positioning actuators are undesirably large and heavy because they employ magnetic circuits having heavy steel yoke elements and magnets to create a concentrated magnetic field of sufficient magnitude to move the objective lens accurately. For an example of such a fine positioning lens actuator, see U.S. Pat. Nos. 4,568,142 and 4,646,283, the complete disclosures of which are incorporated herein by reference. These patents disclose positioning apparatus for an objective lens with, as shown in FIGS. 1A and 1B, a large magnetic circuit having heavy yoke plates (labelled 2a, 2b in FIG. 1A; 4 in FIG. 1B) surrounding a pair of magnets (labeled 3a, 3b in FIG. 1A; 5a, 5b in FIG. 1B). In such a device, the coarse positioning actuator must move a large, heavy payload, resulting in slow seeking operations unless a coarse positioning actuator of substantial size and coarse positioning power is utilized, often limited by internal generation near the optics on the moving part including the objective lens and a turning mirror.
One reason that known devices have incorporated magnetic circuits of such size is that the electro-magnetic coil usage in such devices is generally inefficient. This is due largely to the fact that only a small portion of the coil is active in producing force useful for lens positioning. The nature of the inefficiency is due to having either a poor percentage of the coil present within the magnetic field, or to arranging coils within the magnetic field having a direction of current flow non-orthogonal to the field and desired force axes. In order to generate a positioning force of sufficient strength, the magnetic circuit must generate a relatively large magnetic field to compensate for the inefficiency of the coil. A large magnetic circuit is therefore used to generate a magnetic field which is relatively strong in comparison to the field produced by the coil.
Another important consideration in lens actuator performance is the weight and stiffness of the structural components. In order to achieve a high level of performance, weight should be minimized and stiffness maximized. Typically, the structural members of the fine positioning actuator are created out of aluminum, magnesium or engineering plastics. The problem with these materials is that they may have insufficient rigidity and/or undesirably high weight. The use of conventional materials can therefore result in lower acceleration capability and degraded frequency response affecting seek and track-following performance.
For these reasons, an objective lens fine positioning actuator is desired which has reduced size, height and weight, yet high efficiency in generating magnetic forces to position the lens in both the tracking and focusing directions. It is further desired that the components of the actuator have greater stiffness to improve the actuator frequency response.